IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 2709

Effects of Cu Substitution on Magnetic Properties of Ca�Co�O�

W. Yansen, Agustina Ismail, R. Rajagukguk, C. U. Jung, and B. W. Lee

Department of Physics, Hankuk University of Foreign Studies, Gyeonggi-do 449-791, Korea

We report the magnetic properties of quasi-one-dimensional complex oxides Ca�Co� Cu O� ( � � � �� � ��, and � �). Therhombohedral structure (space group �� ) peaks were well indexed in X-ray diffraction pattern. Ca�Co�O� shows a magnetic transitionat around 27 K, whereas the transition temperature for Ca�Co� �Cu� �O� is around 24 K. The magnetization steps tend to disappear,revealing that they are sensitive to the Cu substitution similar to the manganese or chromium substitution systems. These results indicatethat the Cu�� ions with spin state � � � � are antiferromagnetically coupled to their Co�� neighbors so that the ferromagneticcoupling interaction is reduced.

Index Terms—Crystal structure, magnetic ordering, magnetic properties, X-ray diffraction.

I. INTRODUCTION

T HE COMPOUND of the type Ca Co O crystal-lizing in the rhombohedral structure (space group

) is one-dimensional mate-rial with several interesting properties that have attracted theattention of physicists over the last decade [1]–[4]. Ca Co Ocontains Co O chains along the -axis made of alternatingface-sharing CoO octahedra (Co1) and CoO trigonal prisms(Co2). The Co ions on octahedron site are in low spin (LS)state , and those on trigonal prisms are in high spin (HS)state [5]–[8]. One-dimensionality can be expectedbecause the Co O chains are widely separated by Ca ions.The interchain Co-Co distance is 5.24 , whereas the intrachaindistance is 2.59 . The magnetic moments are and

for octahedral cobalt and trigonal prismaticcobalt ions, respectively [1]. Another interesting point is thata triangular-lattice arrangement of spin chains gives rise tospin frustration from the interaction between Co1 and Co2.It is believed that the intrachain interaction is ferromagnetic,whereas the interchain interaction is antiferromagnetic. Theseantiferromagnetically coupled chains occur via the short O-Obond [9].

Some experiments have proposed that Ca Co O is in fer-rimagnetic state below 10 K and a partially disordered antifer-romagnetic state for K at low magneticfield [1], [2]. For K, the paramagnetic state is observed.This substance exhibits large frequency dependence in ac sus-ceptibility measurement, which is a typical superparamagneticbehavior [10], [11]. However, an earlier report showed the glassbehavior in this compound [12]. In case that Ca Co O is dopedwith Ir ions, the Curie–Weiss temperature increases from28 K to 150 K, suggesting Ir ions are coupled ferromagneticallywith Co in HS state [13]. In this case, they occupyoctahedron sites. However, doping with Fe, the decreasesand Fe ions occupy trigonal prisms, indicating they are coupledantiferromagnetically with Co in LS state [14].

The appearances of a first magnetization plateau in theisothermal magnetization curves for K lead theferrimagnetic ordering [1], [2]. When the magnetic field isincreased for T, the transition from ferrimagnetic to

Manuscript received February 23, 2011; accepted April 27, 2011. Date ofcurrent version September 23, 2011. Corresponding author: B. W. Lee (e-mail:bwlee@hufs.ac.kr).

Digital Object Identifier 10.1109/TMAG.2011.2151848

ferromagnetic state occurs. The magnetization steps exist atlow temperature with increasing magnetic field and becomeobvious at 5 K. These steps tend to disappear when the amountof chromium and manganese substitutions increase [11], [12]. Itsuggests that the intrachain ferromagnetic interaction decreaseswith Cr substitution [11].

Thermoelectric properties of Ca Co Cu O have been in-vestigated and demonstrated the semiconducting behavior, butthe spin state and the occupied site of Cu ions have not beentaken into account [15], [16]. In fact, the spin state and the oc-cupied site of Cu ions are strongly correlated to the magneticproperties of Ca Co Cu O and affect the nature of mag-netic coupling. In this paper, we investigate the effects of Cusubstitution on the magnetic properties of Ca Co O .

II. EXPERIMENTAL

Polycrystalline samples of Ca Co Cu O (, and ) were prepared via solid-state reac-

tion. The reagents CaCO , Co O , and CuO with high puritypowder (99.99%) were thoroughly mixed, pressed into a pelletform, and heated in air at 1000 C for 12 h. Then, the sampleswere ground into powder, pressed into a pellet, and reheated.The samples of doping amount were reheated at1000 C for 48 h in air, whereas was reheated at1000 C for 40 h in a stream of O gas [15], [16].

The structure of samples was confirmed by powder X-raydiffraction (XRD) measurements using Cu radiation. Thelattice parameters were calculated by the least square method.Temperature-dependent magnetization measurements weremeasured in field-cooled (FC) and zero-field-cooled (ZFC)modes using MPMS (3–50 K) and VSM (16–100 K) in thepresence of magnetic field. Isothermal magnetization measure-ments at several temperatures were measured using MPMS.

III. RESULTS AND DISCUSSION

A. Structural Characterizations

Fig. 1 shows the XRD patterns for Ca Co Cu O at roomtemperature. The diffraction peaks were indexed with respect tothe rhombohedral structure with a space group of . The crys-talline size of Ca Co O measured by XRD is around 60.50 nm.

The lattice parameters ( - and -axis lengths) are shown inFig. 2. The -axis length starts to decrease markedly in the rangeof and then increases for . Contrary tothe -axis length, the -axis length increases with increase in

. These observations are in agreement with those reported in

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2710 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

Fig. 1. Powder XRD patterns for Ca Co Cu O .

Fig. 2. Evolution of the lattice parameters versus Cu content.

the literature [15]. In these samples, the partial doping Cu ionin Ca Co O is able to generate the magnetic ordering of Coand Co . The spin states of Cu can be Cu orCu with ionic radii 0.73 and 0.54 , respec-tively [17]. The increase of lattice parameters suggests the Cu(0.73 ) substitutes Co (0.61 for high spin or 0.55 forlow spin). According to Zubkov et al. [18], the Cu and Mn ionsin Ca CuMnO occupy trigonal prisms and octahedron sites,respectively. It was also reported that Sr Cu Ir O containsthe edge-shared Cu-O square planes and Ir-O octahedral. Thesquare plane can be achieved since copper ion displaces fromthe center of trigonal prisms to the rectangular face [19].

B. Magnetic Properties

The temperature-dependent magnetization forCa Co Cu O ( and ) measured ina magnetic field (H) of 100 Oe is shown in Fig. 3. The magnetictransition temperature is determined from the derivationof FC curves measured at 5 kOe (not shown in figure). The

for Ca Co O is observed around 27 K, whereas thatfor Ca Co Cu O is observed around 25 K. Thedecreases from around 27 K for to around 24 K for

(inset of Fig. 3). The FC curve for Ca Co O showsthat the intrachain ferromagnetic nature is more dominant than

Fig. 3. Temperature dependence of magnetization for Ca Co Cu O in thepresence of 100 Oe for ZFC and FC states. Inset shows � dependence ofdoping amount.

interchain antiferromagnetic nature with the magnetizationaround 0.7 emu/g at 5 K. When Ca Co O is doped withCu, the magnetization shown by FC curve decreases fromaround 0.7 to around 0.2 emu/g at 5 K, which indicates thatthe intrachain ferromagnetic interaction becomes weaker.These results reveal that the ferromagnetic coupling interactionis reduced due to the antiferromagnetical coupling of Cuions with Co ions, the equivalent behavior was reported inFe-doped Ca Co O [14], [20]. In contrast, when Ca Co Ois doped with Ir ions, the increases from 28 K to 150 K. Itimplies that Co and Ir are coupled ferromagnetically andthe ferromagnetic intrachain interaction increases [13]. TheZFC and FC curves for Ca Co Cu O exhibit a largebifurcation at , which is around 10 K, and small bifurcationin the range K, then coincide at K.This bifurcation might be due to freezing of random spins [20],but further experiments are needed to prove this analysis. Thedecrease of was also found on Ca Co O doped with sometransition metals such as Mn and Fe [12], [20].

Fig. 4(a) shows the isothermal magnetization result forCa Co O . Below 30 K, the magnetization increases abruptlyat low magnetic field, and the plateau (1/3 of the saturatedmagnetization) is observed for K at certain magneticfields, suggesting a ferrimagnetic state since two of the threeCo-O chains in the hexagonal unit cell are coupled antiferro-magnetically [1], [2], [11]. The ferromagnetic state is observedat high magnetic field ( T). These results are similarto other literature [1], [2]. The schematic phase diagram forCa Co O has proposed that below 10 K with certain mag-netic fields, the ferrimagnetic state changes to spin freezingphase [2]. The hysteresis curve and steps appearing at 5 K withincreasing magnetic field suggest that the stability of manyenergetically close magnetic structures are sensitive to themagnetic field [12]. The magnetic field at which the transitionfrom ferrimagnetic state to a ferromagnetic one occurs isdefined as characteristic magnetic field . Thedecreases from 3.5 T at 5 K to 3 T at 20 K. At K, themagnetization is proportional to the magnetic field, showingthe paramagnetic state.

Magnetization versus external magnetic field for sampleis shown in Fig. 4(b). The plateau is also observed

YANSEN et al.: EFFECTS OF Cu SUBSTITUTION ON MAGNETIC PROPERTIES OF Ca Co O 2711

Fig. 4. Isothermal magnetization for (a) Ca Co O and(b) Ca Co Cu O as a function of magnetic field (� � � � ��� T) atselected temperatures. The vertical arrows mark steps in the curve.

for K. The decrease of appears clearly at15 K from 3.5 T to 3 T , indicating thatthe ferrimagnetic state decreases. The hysteresis curve at 5 Kfor Ca Co Cu O is smaller than that of Ca Co O , andthe value of plateau in magnetization at 10 K also decreasesfrom around 0.88 /f.u. to 0.8 /f.u. for Ca Co O andCa Co Cu O , respectively. It suggests that Cu ionsubstitutes Co ion. This substitution induces the magneticmoments of Cu coupled antiferromagnetically with Coand breaks the intrachain ferromagnetic interaction or disturbsthe magnetic structure of Ca Co O and leads to disorderof the magnetic structure so that the saturated magnetizationdecreases. The magnetization steps also tend to disappearwhen doped with Cu. It implies that the magnetization stepsare sensitive to the Cu substitution. It has been proposed thatthe steps are due to quantum tunneling of the magnetizationor the magnetic field induced transitions between differentconfigurations on triangular lattice [11], [21].

IV. CONCLUSION

The increase of lattice parameters suggests the Cu substi-tutes Co . The magnetic properties of Ca Co O are very sen-sitive to the Cu doping, which is indicated by the lower magnetictransition point and the smaller hysteresis curve at low temper-ature than the undoped one. The decrease of from around27 K for to around 24 K for indicates thatthe intrachain ferromagnetic interactions decrease with Cu sub-stitution, suggesting antiferromagnetical coupling of Cu ionswith Co ions. Further research is needed to investigate thecrystal structure and magnetic properties in the antiferro- andferromagnetic states of Ca Co Cu O .

ACKNOWLEDGMENT

This work was supported by Mid-career Researcher Pro-gram (20090084750) and Basic Science Research Program(20090066917) through an NRF grant funded by the MEST.

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